1. Field of the Invention
This invention relates to radio receivers and, more particularly, to impulse reduction within frequency modulation (FM) receivers.
2. Description of the Related Art
Frequency modulation (FM) radio is widely used in many applications. One reason FM became more popular for music broadcasts than, for example, amplitude modulation (AM), is in many cases FM reception is less affected by noise than AM. In an FM signal, the transmitted carrier signal frequency is varied by the frequency of the information (e.g., at an audio rate) within a given bandwidth. In an AM signal, the transmitted carrier signal amplitude is varied by the frequency of the information (e.g., at an audio rate), thereby creating an envelope that is defined by the information frequency. As such, a significant drawback to AM signals is the amount of noise that may be present in the received signal. In many FM receivers, the effects of received noise may be reduced using various well-known methods such as limiting or over-driving. Since noise in an FM signal may be “riding” on the peaks of the received signal, much of the noise may be removed, while the information carried by the signal remains.
However in low signal level environments, the difference between the noise and the signal may become increasingly smaller (e.g., low signal-to-noise ratio). In FM, the instantaneous frequency includes the transmitted information. Many FM receivers demodulate the FM signal by obtaining the instantaneous frequency of the signal by differentiating the phase of the signal. At low signal levels, an increase in the noise (especially additive noise) in the received signal may cause the output of the demodulator to include unwanted impulses.